It is recognized that the overall fuel consumption efficiency of a wheeled motor vehicle is based on its weight, its frontal area and its rolling resistance. Since rolling resistance is determined by the tires of the vehicle, much effort has been directed to lowering the rolling resistance of pneumatic `radial` automobile and truck tires, as for example disclosed in U.S. Pat. No. 4,281,703, which teaches that a particular tread composition on a conventional radial tire produces reduced rolling resistance.
Other patents, such as U.S. Pat. No. 4,224,197 and British (U.K.) patents Nos. 2,057,455 and 2,083,486 disclose the importance of providing a tire with a tread having low rolling resistance. The '455 U.K. patent teaches that lowering rolling resistance without adversely affecting the wet or dry traction of the tire is a difficult task.
A major reinforcing agent for rubber tread vulcanizates such as in automobile and truck tires, is carbon black. Carbon black greatly enhances mechanical strength and resistance to abrasion and is thus widely used. However, the availability of fine-particle reinforcing types of carbon black, including channel black, high abrasion furnace black (HAF), super abrasion furnace black (SAF), and intermediate super abrasion furnace black (ISAF), is petroleum-dependent, all being made by the thermal decomposition of hydrocarbons.
Concern over the availability of petroleum initiated several programs to evaluate numerous nonblack reinforcing fillers. The best nonblack fillers developed to date are the precipitated fine-particle silicas. In an article titled "Fine-Particle Silicas in Tire Treads, Carcass, and Steel-Belt Skim" by Wagner, M. P., published in Rubber Chemistry and Technology, Vol. 50, No. 2, May-June 1977, it was generally demonstrated that reinforcing silicas can be considered as alternates to contemporary fillers.
However, as pointed out in U.S. Pat. No. 4,150,014, vulcanizates obtained from silica or silicate filled polymers are known to be `boardy`, which may be described as a stiffness at low elongations and lack of resilience, and due to high tensile set, both of which deficiencies can be avoided in carbon black filled vulcanizates. Despite numerous efforts to resolve the problem satisfactorily, to date, we know of no silica-rich tread vulcanizate with acceptable strength, abrasion, resilience and tensile set properties which tread is the critical component of a pneumatic tire with reduced rolling resistance without substantial loss of wet or dry traction, irrespective of whether or not the silica-rich tread is especially formulated for low rolling resistance. By "silica-rich" we refer to a tread which contains at least 1 part hydrated amorphous fine particle silica per 3 parts by weight of carbon black filler. Further, we do not know of a silica-rich tread which can be used in a tire having reduced rolling resistance with substantially no loss of wet or dry traction, and the tire still produced at reasonable cost. This is in part due to the problem of selecting an acceptable silane coupling agent, and having made a suitable choice, incorporating the silane into a tread compound without deleteriously affecting workers in the plant. As is well known, mercaptosilane coupling agents, which have been found to be peculiarly suited for silica-filled tread vulcanizates, are overpoweringly malodorous.
The use of various silicon-containing coupling agents to enhance the adhesion of various inorganic substrates with a broad variety of organic polymers to promote coupling and bonding therewith is well known in the art. Examples are provided in U.S. Pat. Nos. 2,832,754; 2,971,864; 3,258,477; 3,661,628; 3,671,562; 3,705,911; 3,706,592; 3,754,971; 4,151,157 inter alia. As is conventionally understood, the silicon coupling agent serves as a cross-linker that is chemically or physically bonded to both the inorganic filler or other substrate, and the organic polymer in the polymer composite.
Among the numerous silane coupling agents deemed useful, it was found that those with a polysulfide structure are particularly well-adapted for the vulcanization of silica reinforced rubber compositions, particularly earthmover tire treads which require high tensile and tear properties, high roadwear index and low heat build-up. See Reinforcing and Vulcanization Effects of Silane Si 69 in Silica-Filled Compounds", presented at the Australasian Rubber Convention in Canberra, by Siegfried Wolff of Degussa Corporation in 1980. It was therefore quite unexpected that a mercapto-substituted silane in a silica-rich rubber tread would provide comparable physical properties as those disclosed for the Si 69 polysulfide, and in addition, provide a tire having a tread with reduced rolling resistance without loss of traction, both of which properties are determinative of the successful performance of a high quality automobile or truck tire.
A mercaptosilane coupling agent for a silica-filled SBR has been disclosed in an article titled Precipitated Silicas--A Compounding Alternative with Impending Oil Shortages", by Wagner, M. P. in Elastomerics, August 1981, pgs 40 et seq. However, there is no suggestion as to what effect the vulcanized tread may have on rolling resistance or on dry traction. Most importantly, there is no suggestion as to how a mercaptosilane may be incorporated into a tread compound on other than a laboratory scale, because of the problems of handling the mercaptosilane.
The problem of odor from a mercaptosilane was recognized and solved in U.S. Pat. No. 4,184,998 by reversibly blocking the mercapto group by a simple addition reaction. This converts the normally liquid mercaptosilane which is relatively easily vaporized under compounding conditions, into an essentially non-volatile solid free from objectionable odor. It is also there disclosed that reinforcing grades of silica may be used as fillers in amounts as high as 17 parts silica per 40 parts of super abrasion furnace black mixed with 100 parts rubber, but there was no reason to expect that a tread compound containing such a silica in a wide range from as little as 5 parts to as much as about 75 parts by weight per 100 parts rubber, when vulcanized with the reversibly blocked mercaptosilane, would produce a tread with excellent dry traction and reduced rolling resistance.